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Do I need a spacer or two?
Possibly, if the final sprocket doesn't clear the end of the freehub, then yes. Usually one is enough
Yes. You need a 1.85mm spacer. If your cassette is 105, ultegra or dura ace you'll also need a 1mm spacer.
Thanks I just put 1 on and it’s not behaving. I’ll throw another on thankyou
Not behaving in what way?
While you can just chuck random spacers at it there are specific ones designed to do the job that just work. They'll put the cassette in the correct position to give proper thread engagement for the lockring.
11spd freehub (which you have) with 10spd cassette requires a 1.85mm spacer.
Certain Shimano 10spd cassettes - 105 (discontinued), ultegra and Dura ace also need a 1.0mm spacer (which they come with). Current Shimano 10spd cassettes (eg hg500) don't require this 1mm spacer, neither do SRAM 10spd cassettes.